Frost on grass. Credit: jason english/EyeEm

White frost visible on MSG RGB composites

24 July 2009 00:00 UTC

Frost on grass. Credit: jason english/EyeEm
Frost on grass. Credit: jason english/EyeEm

Two frost events were identified with Meteosat-9 imagery during the austral winter in 2009.

Last Updated

07 June 2022

Published on

24 July 2009

By Lynette van Schalkwyk (South African Weather Service)

See map of South Africa , the areas where frost occurred are indicated with red ovals.

Case 1: Frost on the eastern Highveld of South Africa on 24 July 2009

A strong ridge of the Atlantic Ocean high pressure system resulted in marginally unstable and clear conditions over the eastern Highveld of South Africa during the early morning hours of 24 July 2009 (see surface chart, the area where white frost occurred is indicated with a red oval). A bluish hue appeared on the Day Natural Colours RGB image immediately after sunrise, together with bright cyan areas, indicating fresh snow on the eastern and southern borders of Lesotho (Figure 1).

An animation (05:15–06:45 UTC, AVI) of the images after sunrise shows the snow covered mountains unchanged, while the white frost quickly melted away as temperatures increased. By making use of the RGB VIS0.8, NIR1.6, IR3.9r composite (Day Fog-Snow), the area where white frost occurred shows up in a red-orange colour (see animation, 05:15–06:45 UTC, AVI). Observations at 03:00 UTC and 06:00 UTC indicate temperatures below 0 ºC in the area where frost was observed (indicated by red oval in the surface chart).

 

White frost visible on MSG RGB Composites
Figure 1: Meteosat-9 Natural Colour RGB, 24 July 2009, 05:45 UTC. Large Area.
Animation (05:15–06:45 UTC)

Case 2: Frost in the southern Freestate and northern Eastern Cape on 20 August 2009

With a cold front that passed to the east of the country, and snowfalls that occurred on the high lying areas of the Eastern Cape and Lesotho (indicated by yellow arrows in Figure 2), a high pressure system remained dominant over the central interior of South Africa on 20 August 2009. In addition to the snow, widespread white frost appeared in a red-orange colour on the Day Snow-Fog RGB (indicated with red ovals, see corresponding animation, 06:00–07:45 UTC).

The white frost can also be seen on the Day Natural Colours RGB (see animation, 06:00–07:45 UTC). Shortly after sunrise the frost started melting as temperatures started to increase. Observations at the time of frost occurrence indicate temperatures in the area below 0ºC with light winds during the early morning. The occurrence of frost was confirmed by frost reports at 06:00 UTC of weather observers at Bloemfontein and De Aar weather offices.

White frost visible on MSG RGB Composites
Figure 2: Meteosat-9 Snow-Fog RGB, 20 August 2009, 05:45 UTC. Large Area
Animation (06:00–07:45 UTC)

Additional content

White frost in Hungary on 29 November 2007 (Animation Met-9 Day Snow-Fog RGB (enhanced), 07:00–10:00 UTC, Source: M. Putsay)